Comment # 7 on bug 1228849 from pallas wept

Aha, you've spotted the problem! This vm lives in a btrfs subvolume inside my
home subvolume, ~/Documents/_NoCoW which as the name suggests, has
copy-on-write disabled, and is a subvolume rather than a normal directory in
order to partition it from my snapshots. A subdirectory beneath that, seemed
like a good place for VMs to live...

The whole thing is missed by the autorelabel. I started to search for info
about this before I reported it, and I gather that it is known behaviour that
btrfs subvolumes are missed, but I don't know what to do about it yet. 

If you have any advice about how I should handle that, it'd be appreciated. I
understand if this isn't a supported configuration. Thanks for finding the
problem for me, and I hope I haven't wasted your time with this bug.